The present application is directed to an expandable interbody device which is inserted from a posterior approach between adjacent vertebrae in the spine of a patient and which is also selectively anteriorly expandable.
Fusion cages, as well as other related interbody devices, are frequently utilized in spinal surgery between vertebrae of a patient. In particular, one or a pair of interbody devices are placed between the vertebrae to provide support and promote fusion between vertebrae where such is necessary due to disease, injury, general deterioration or a congenital problem.
Frequently, the anterior sides or fronts of the vertebrae also require additional spacing in comparison to posterior sides to correct curvature of the spine. Therefore, it is often desirable to use an anteriorly expandable interbody device so that the vertebrae are spread or spaced more on the anterior sides thereof than on the posterior sides thereof. It is seldom if ever desirable to space the posterior sides of the vertebrae more than the anterior sides thereof. Interbody devices which provide for anterior greater expansion are generally referred to as interbody expansion devices or expansion fusion cages. They are specifically expandable on the anterior end thereof such that the fronts of the adjacent vertebrae are more greatly spaced than the rears.
It is noted that interbody devices, such as fusion cages, may be inserted into the intervertebral space anteriorly or posteriorly. That is, in some instances the surgery is performed from the front and sometimes from the rear of the patient. The present application is directed to types of devices that are inserted posteriorly or from the rear of the patient and are generally referred to as posterior interbody devices or posterior fusion cages.
Interbody devices typically must be very strong along the entire length of the top and bottom walls of the device that engage the vertebrae. In particular, in the turning and twisting of everyday life, substantial forces can be exerted against the interbody devices and, in particular, against the anterior end thereof. This is even more so when the devices are subjected to unusual forces during an accident or the like. Some types of interbody devices in the prior art have provided for anterior expansion, but have not well supported the anterior end of the upper and lower walls. Such prior art devices have provided some kind of a wedge or rod that is urged rearward along a ramp of the device in such a way as to expand the anterior portion subsequent to it being placed between the vertebrae. Because the wedge moves rearwardly or posteriorly toward the back wall of the device in order to spread the walls, a lever arm of unsupported wall is formed between the front edge of each wall and the position where the wedge engages the walls. Substantial forces can act on this lever arm. The interbody devices poorly supported along the unsupported wall can fail due to forces exerted along the lever arm in the region.
Secondly, it is desirable for the interbody devices to not expand sideways, while the devices are expanding vertically. This need arises because it is often necessary to put interbody devices in close proximity to one another in side by side relationship. Sideways expansion may prevent desirable positioning of the interbody devices and may also interfere with positioning of bone chips for grafting between the interbody devices. Some prior art devices expand radially which expands the devices vertically, but also sideways.
When installing interbody devices, such as fusion cages, a surgeon may also find that it is necessary to have more anterior expansion than was initially anticipated in order to correct spinal curvature. Consequently, it is often desirable to be able to increase the expansion in increments that allow the surgeon to determine whether expansion is sufficient after each incremental advance in expansion and then further increase the expansion, if necessary. Therefore, it is desirable to have a kit that provides for alternative expansion devices, especially where such modular expansion devices can each be screwed into and secured in place in the cage so as to resist inadvertent removal during use.
Interbody devices, such as fusion cages, are quite expensive to produce in general. This is because the body of the devices must be made to very high tolerances in order to provide reproducible results and to provide the strength necessary to support the spine of the person. Because the surgeon is not always certain exactly which expansion size interbody device will be required until the surgery site is opened, it has been necessary for the surgeon to have on hand many different sizes of fusion cages and, in some cases, different types of fusion cages, such as expandable and non-expandable, so as to insure that the necessary item will be present when the surgery is performed.
Consequently, it is also desirable to be able to provide an interbody device that is modular in nature and easily adjusted to many degrees of expansion and that it may be used as efficiently with no expansion as with expansion, so that the same body can be used with different degrees of expansion. Many of the expandable types of prior art fusion cages cannot be used as non-expandable type cages.
Further, it is desirable for a single fusion cage body to be utilized for virtually any degree of expansion desired or foreseeable by simply providing a set of comparatively much less expensive expansion members in a kit, any of which expansion members may be used in conjunction with the body to provide for various and different degrees of expansion and each of which may be removed after insertion into a fusion cage body and replaced with a different expansion member without removing the cage body from between vertebrae.
A posterior interbody device or fusion cage for use between a pair of adjacent vertebrae in order to provide support and/or promote growth between the vertebrae that have been destabilized due to injury, illness or the like. The interbody device includes a body which may be rectangular in shape in which case the device is slid or driven between the vertebrae or generally cylindrical in shape and often threaded, in which case the device is screwed between the vertebrae. The body has an upper wall and a lower wall that are connected by a rear or posterior wall in a somewhat U-shaped configuration. The body is hinged about the rear wall by utilization of a material of construction that is flexible, such as stainless steel or titanium, so that the rear wall functions like a spring in conjunction with the upper and lower walls, so that the upper and lower walls are able to pivot relative to each other at the rear, when a spreading force is applied to the walls, which causes the anterior ends of the upper and lower walls rotate from an initial non-expanded configuration anteriorly to an expanded configuration anteriorly. In particular, the upper and lower walls are initially in a non-expanded state wherein the upper and lower walls are generally parallel and subsequently may be expanded by a plurality of expansion members to various expanded states wherein the upper and lower walls are at angles relative to each other which angles increase with expansion.
The upper and lower walls also have legs on either side of the anterior end thereof that face toward similarly positioned legs on the other wall and which abut against each other when the device is in the non-expanded configuration thereof, so the cage can be utilized operably in a non-expanded configuration. An aperture is formed between the legs and preferably extends through the body in such a manner as to form an interior chamber suitable for receiving bone chips or other growth promoting media. The body also preferably has upper and lower windows which communicate with the chamber and open onto the surface of the vertebrae, when in use, so as to promote growth of bone through the interbody device. The rear wall of the body includes a threaded bore.
An expansion member, preferably having a shape similar to a large headed bolt, is utilized to apply spreading force to the upper and lower walls so as to expand the body anterior end. The expansion member includes an elongate shaft having a rear portion threaded so as to be operably and threadably received in the rear wall bore and has a head at an opposite end. The shank includes a stop, preferably adjacent the threaded rear portion abutting the rear wall and being of enlarged diameter compared to and adjacent to the threaded rear portion, which insures that the expansion member is properly positioned during use.
The expansion member head varies in diameter depending on the expansion desired. The expansion head is sized, and shaped in position so as to engage a wedge mating or ramp surface located anteriorly on facing surfaces of each of the lower and upper walls. In particular, the expansion head first engages the mating surfaces on the walls near the posterior end of the ramp surface and then the mating surface slides along the ramp surface as the expansion member is screwed into the rear wall bore. Alternatively, the ramp may be associated with the wall and the expansion member then has an edge or surface that mates with and slides along such a ramp in the wall. In this manner the anterior end of the body is forced to spread or space vertically until a forward or anterior end of the ramp surface is reached. The ramp surface is adjacent to a support surface that is generally parallel to an axis of rotation of the expansion member and transfers the support of the upper and lower walls to the support surface as the expansion member is further rotated clockwise and advanced into the rear wall bore. Continued advancement of the expansion member to the stop causes the head to pass between the anterior ends of the upper and lower walls and to be fully supported by opposed surfaces to thereafter provide support to the upper and lower walls at the anterior ends thereof and keep the upper and lower walls in a preselected spaced relationship relative to each other.
Finally, in use each interbody device is normally provided with a set or kit of expansion members wherein each member of the set provides a different degree of spacing of the anterior end of the interbody device, for example, with one half or one millimeter differences in spacing between each size. In this manner, a surgeon can utilize the interbody device without an expansion member or can alternatively select from a number of expansion members with different sized heads to provide appropriate expansion of the anterior end of the interbody device. Normally, the surgeon would start with no or minimal expansion and then increase incrementally toward greater expansions until the surgeon is satisfied with the expansion provided.
Therefore, the objects of the present invention are: to provide a posterior interbody device or fusion cage for use between a pair of vertebrae that is expandable; to provide such an interbody device that has a body and at least one expansion member wherein the expansion member includes a head that engages an anterior portion of upper and lower walls of the body so as to spread the body from the anterior end thereof; to provide such a device wherein a single body may be utilized either without an expansion member or in conjunction with any of an alternative group of modular expansion members, each producing a different degree of expansion and contained in a kit of expansion members; to provide such a device wherein an expansion member that produces one degree of expansion can be screwed into the body such that the body expands and thereafter the expansion member can be removed and another expansion member producing a greater degree of expansion can be subsequently inserted; to provide such a device wherein the expansion members include an anterior or frontward head that rests on opposed anterior surfaces of the upper and lower body walls subsequent to full insertion of the expansion member; to provide such a device wherein the device expands vertically and not horizontally or side to side; to provide such a device including a central cavity and windows to allow for packing with bone chips or other growth media so as to promote fusion between adjacent vertebrae exposed to the windows; to provide such a device wherein a single body may be alternatively utilized with a number of different expansion members, such that multiple different sized bodies are not required to be maintained in stock during a surgical operation; to provide such a device that does not cantilever the walls over a wedge that is medially located with respect to the body, but rather positions the expansion head, ramp surfaces and the support surfaces at or near the anterior end of the body at all times during expansion, so as to continuously provide support to anterior ends of the walls; and to provide such a device which is relatively inexpensive to produce, extremely easy to use and especially well adapted for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.